1. Field of the Invention
This invention relates to rotary extruders, and more particularly, to a method and apparatus for monitoring pressure within a rotary extruder and for selectively controlling the operation of either a melt pump disposed downstream of the extruder outlet or a feed mechanism supplying material to the extruder based upon pressure deviations within the extruder.
2. Description of the Prior Art
As used herein, the term "rotary extruder" refers to an extruder having a stationary barrel section with an eccentric bore and a rotor that comprises a substantially cylindrical plasticizing surface rotatably mounted within the bore. Polymeric resin, typically in the form of pellets, flakes or powder, is introduced through a feed port into a clearance zone between the rotor and the interior surface of the barrel. As the rotor rotates within the barrel, the resin is plasticized and delivered to a discharge port. Plasticized material exits the barrel and is then usually further pressurized by a melt pump or equivalent means and extruded through a die.
Extruders of this general type, which differ substantially from conventional rotating screw-type extruders in their design and operation, have already been disclosed, for example, in U.S. Pat. Nos. 3,880,564; 4,012,477; 4,501,543; and 4,813,863.
Unlike most conventional screw extruders, rotary extruders are not flood fed, and weigh-belt feeders have been utilized previously in an effort to maintain a constant feed rate. Such rotary extruders have been operated by adjusting the feed rate and the melt pump rate to achieve a desired throughput and then adjusting the speed of the rotor to plasticize that amount of material. In some instances, however, difficulties have been encountered in operating the rotary extruder in this manner.
The average residence time required to plasticize a polymer within a rotary extruder is significantly less than that typically required to plasticize the same polymer in a conventional screw extruder, thereby minimizing the time during which the polymer may be subjected to shear. Due to differences in the internal design of the rotary extruder, however, the hydraulic pressure in the annular space between the rotor and barrel does not always vary in the manner that one might expect when operating a screw extruder.
It has been discovered, for example, that when overfeeding occurs in a rotary extruder operated according to conventional methods, pressure may rise initially but then drop suddenly if the overfeeding is not quickly corrected. Assuming that the melt pump continues to operate at the same speed, the amp load on the rotor drive motor increases in response to the increased amount of unplasticized material within the extruder. As the amount of work being done by the rotor increases, the amount of shear exerted on the polymer also increases, melt viscosity decreases, and melt pressure within the extruder drops. This pressure drop within the extruder barrel as a result of overfeeding is contrary to prior experience with screw extruders.
Several methods and apparatus have previously been disclosed for use in monitoring and controlling melt pressure in screw-type extruders.
U.S. Pat. No. 2,747,222, for example, discloses the use of a bellows in a chamber below the extruder die outlet to sense pressure variations and adjust the speed of pultrusion rollers when pressure limits such as plus or minus 100 psi are exceeded.
U.S. Pat. No. 2,747,224 discloses a pressure control system for a screw-type extruder wherein the speed of the screw is regulated according to the pressure of the plasticized material at or just beyond the delivery end of the screw. With such an extruder and control system, however, melt pressure is increased by increasing the screw speed, which in turn increases the amount of feed entering the barrel. Thus, the melt pressure and the amount of feed are directly proportional, unlike the situation referred to above which has been experienced during operation of the rotary extruder.
U.S. Pat. No. 3,283,041, discloses a method and apparatus for monitoring back pressure in the barrel of a screw-type extruder and for increasing back pressure by cooling the nose of the screw. Plus or minus pressure deviations with respect to a specified set point are used to control a valve that restricts the flow of a cooling medium through the screw. U.S. Pat. No. 3,283,041 also discloses other methods previously utilized to control hydraulic pressure within conventional extruders, including the use of different feed screws to achieve different pressures; the use of flow restrictions or valves at the outlet orifice; and cooling the barrel around selected portions of the screw.
U.S. Pat. No. 3,357,049 discloses a method and apparatus for monitoring and controlling melt pressure in a chamber downstream of an axially reciprocable, rotatable extruder screw through use of a piston adapted to vary the width of the gap between a torpedo disposed on the end of the screw and an annular ring.
U.S. Pat. No. 4,290,986 and 4,197,070 disclose another method and apparatus for monitoring melt pressure near the output end of the barrel of a conventional screw-type extruder, and for maintaining the melt pressure at a desired level by controlling the speed of the extruder screw.
Notwithstanding the methods and apparatus previously disclosed in the prior art, a control system is needed that is useful for monitoring and controlling the melt pressure within a rotary extruder.